One method of forming a helically-shaped (coil) spring is by winding a wire feedstock heated to annealing temperature. Subsequent to cooling of the formed coil spring, the spring can be heat-treated, for example, by reheating to a suitable tempering temperature.
One method of heating a coil spring by electric resistance heating is described in U.S. Pat. No. 6,235,131 B1. In the disclosed method, connectors (38, 40) are connected to a suitable source of electric current, and are located remote from the free ends of the coil spring so that the spring is hardened, or tempered, by electric resistance heating in the coiled section between the connectors to a greater degree than that at the free ends of the spring.
Another method of accomplishing the tempering by electric resistance heating is described in U.S. Pat. No. 2,261,878. In the disclosed method, one extended end region of a spring 116 is placed in contact with a fixed plate-type electrical contact (electrode) 121 while the spring is compressed by a moveable plate-type electrical contact (electrode) 120 at its opposing extended end region as illustrated in FIG. 1(a) herein. Both fixed and moveable contacts are connected to a suitable source of electric current to heat treat the entire spring. A disadvantage of this method is that a significant extended end region of the spring makes partial physical contact with either electrical contact, for example, spring end region 116(a) and contact 120, as shown in FIG. 1(b). This arrangement does not generally establish a uniform cross sectional current density in the extended end regions of the spring, which can result in inadequate tempering in the extended end regions of the spring.
It is one objective of the present invention to temper, or otherwise metallurgically heat treat, a helical spring, or a similar article of manufacture, along its entire length while maintaining a substantially uniform cross sectional current density along the entire length of the spring that includes the ends of the spring, or the similar article of manufacture.